<PAGE>
 
                                                                  EXECUTION COPY
                                                                  --------------


                               AMENDMENT NO. 1 TO
                      AGREEMENT AND PLAN OF REORGANIZATION
                      ------------------------------------


     AMENDMENT NO. 1 (this "Amendment") dated as of March 18, 1999, effective
                            ---------                                        
for all purposes and respects as of January 1, 1999, to Agreement and Plan of
Reorganization dated as of August 15, 1997 (the "Original Agreement") among
                                                 ------------------        
VISTANA, INC., a Florida corporation ("Vistana"), VISTANA WEST, INC. (f/k/a V
                                       -------                               
Sub-1, Inc.), a Florida corporation and a wholly-owned subsidiary of Vistana
                                                                            
("VS1"), DONALD J. DUBIN ("Dubin"), LARRY D. DOLL ("Doll"), RONALD R. SHARP
-----                      -----                    ----                   
("Sharp"), DAVID E. BRUCE ("Bruce") and DAVID H. FRIEDMAN ("Friedman")
-------                     -----                           --------  
(capitalized terms not otherwise defined in this Amendment are used herein as
defined in the Original Agreement);


                              W I T N E S S E T H:
                              - - - - - - - - - - 

     WHEREAS, the parties hereto desire to amend the Original Agreement pursuant
to Section 8.6 thereof upon the terms and conditions hereinafter set forth.

     NOW, THEREFORE, in consideration of the premises and the mutual promises
contained herein, and for other good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto, intending
legally to be bound, hereby agree as follows:

     1.   Amendments to Original Agreement.  Schedule 1.5A to the Original
          --------------------------------   -------------                
Agreement is hereby amended in its entirety by substituting attached Schedule
                                                                     --------
1.5A (Amended) therefor.
--------------          

     2.   Miscellaneous.
          ------------- 

          (a) Full Force and Effect.  Except as expressly set forth herein, this
              ---------------------                                             
Amendment does not constitute a waiver or modification of any provision of the
Original Agreement.  Except as expressly amended hereby, the Original Agreement
shall continue in full force and effect in accordance with the provisions
thereof on the date hereof.  As used in the Original Agreement, the terms "the
Agreement," "herein," "hereof," "hereinafter," "hereto" and words of similar
import, shall, unless the context otherwise requires, mean the Original
Agreement as amended by the Amendment.  References to the terms "Agreement"
appearing in the Exhibits or Schedules to the Original Agreement, shall, unless
the context otherwise requires, mean the Original Agreement as amended by this
Amendment.

          (b) Headings and terms.  The headings in this Amendment are for
              ------------------                                         
purposes of reference only and shall not be considered in construing this
Amendment.  Terms defined in the singular shall have a comparable meaning when
used in the plural, and vice versa.
<PAGE>
 
          (c) Counterparts.  This Amendment may be executed in any number of
              ------------                                                  
counterparts, each of which when so executed and delivered shall constitute an
original and all together shall constitute one agreement.

          (d) Law Governing.  THIS AMENDMENT SHALL BE CONSTRUED AND ENFORCED IN
              -------------                                                    
ACCORDANCE WITH AND SHALL BE GOVERNED BY THE LAWS OF THE STATE OF FLORIDA,
WITHOUT GIVING EFFECT TO ITS CONFLICT OF LAWS PROVISIONS.

          IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the date first above written.
 
                              VISTANA:
                              ------- 

                              VISTANA, INC., a Florida corporation


                              By:    /S/ Jeffrey A. Adler
                                 ---------------------------------
                                Name:    Jeffrey A. Adler
                                Title:   President and Co-Chief Executive
                                         Officer

                              VS1:
                              --- 

                              VISTANA WEST, INC., a Florida corporation



                              By:    /S/ Charles E. Harris
                                 ---------------------------------
                                Name:    Charles E. Harris
                                Title:   Senior Vice President



                              DUBIN:
                              ----- 



                                      /S/ Donald J. Dubin
                                 ---------------------------------
                                    Donald J. Dubin


                    [SIGNATURES CONTINUED ON FOLLOWING PAGE]

                                      -2-
<PAGE>
 
                              DOLL:
                              ---- 



                                      /S/ Larry D. Doll
                                 ---------------------------------
                                    Larry D. Doll


                              SHARP:
                              ----- 



                                      /S/ Ronald R. Sharp
                                 ---------------------------------
                                    Ronald R. Sharp
 

                              BRUCE:
                              ----- 



                                      /S/ David E. Bruce
                                 ---------------------------------
                                    David E. Bruce


                              FRIEDMAN:
                              -------- 



                                      /S/ David H. Friedman
                                 ---------------------------------
                                    David H. Friedman



                   [SIGNATURES CONTINUED FROM PRECEDING PAGE]
<PAGE>
 
                                                                  EXECUTION COPY
                                                                  --------------

                                 Schedule 1.5A
                                   (Amended)


          All capitalized terms used in this Schedule 1.5A but not defined
                                             -------------                
herein shall have the meaning ascribed to them in the Agreement and Plan of
Reorganization dated as of August 15, 1997, as amended by Amendment No. 1
thereto dated as of March 18, 1999 and effective as of January 1, 1998 (as
amended, the "Agreement") among VISTANA, INC., a Florida corporation, VISTANA
              ---------                                                      
WEST, INC. (f/k/a V Sub-1, Inc.), a Florida corporation and a wholly-owned
subsidiary of Vistana, DONALD J. DUBIN, LARRY D. DOLL, RONALD R. SHARP, DAVID E.
BRUCE and DAVID H. FRIEDMAN.

          Section 1.  The shares of Vistana Common Stock to be delivered
          ----------                                                    
pursuant to Section 1.5(a) of the Agreement (the "Contingent Shares") are
                                                  -----------------      
subject to delivery by Vistana depending upon the Net Sales (as hereinafter
defined) and the Cost of Sales Percentage (as hereinafter defined) achieved by
the Limited Liability Companies (other than SD), SCI, DMA and SWC (collectively,
the "Marketing Entities") during the three-year period beginning January 1, 1998
     ------------------                                                         
and ending December 31, 2000 (the "Earnout Period").  The Contingent Shares to
                                   --------------                             
be delivered pursuant to the POC Stock Purchase are sometimes herein referred to
as the "POC Contingent Shares" and the Contingent Shares to be delivered
        ---------------------                                           
pursuant to the SCI Stock Purchase and the DMA/SWC Stock Purchase are sometimes
herein referred to as the "Success Contingent Shares".
                           -------------------------  

          Section 2.  Various levels of Net Sales and various ranges of Cost of
          ----------                                                           
Sales Percentages for each calendar quarter and calendar year during the Earnout
Period are set forth in Schedule A attached hereto under the headings "Total Net
                        ----------                                              
Sales" and "Range of Costs," respectively, along with the related percentage of
the Contingent Shares earned in any applicable period based upon the actual
level of Net Sales and actual Cost of Sales Percentage achieved (the number of
Contingent Shares earned in any applicable period or periods, in the case of
cumulative analysis, is referred to as an "Outcome").  For purposes of Schedule
                                           -------                     --------
A, it is understood that the percentages apply to the amount of Contingent
-                                                                         
Shares allocable to such period and not the total amount of Contingent Shares
for the Earnout Period.  For these purposes, one-third (1/3) of the POC
Contingent Shares and one-third (1/3) of the Success Contingent Shares shall be
allocated to each calendar year during the Earnout Period.

          Section 3.
          ----------

          (a) The parties shall determine the number of Contingent Shares earned
by the Selling Parties on a quarterly basis (with twenty-five percent (25%) of
the total number of Contingent Shares for each calendar year being allocated to
each calendar quarter of such calendar year); provided, however, that for each
                                              --------  -------               
calendar quarter of a calendar year (other than the first calendar quarter) the
determination of the number of Contingent Shares earned shall, subject to
Section 3(d) hereof, be based upon the relevant Outcome for the actual
cumulative Net
<PAGE>
 
Sales for the calendar year to date and the actual Cost of Sales Percentage for
the calendar year to date.

          Example:  Subject to Section 3(d) hereof, if the actual Net Sales for
          -------                                                              
     the six-month period ending June 30, 1998 are at least $14,811,402 but less
     than $15,551,972, the Selling Parties shall have earned (i) 100% of the
     Contingent Shares allocable to the first two quarters of 1998 so long as
     the actual Cost of Sales Percentage is less than 46.0%; (ii) 95% of the
     Contingent Shares allocable to the first two quarters of 1998 so long as
     the actual Cost of Sales Percentage is greater than 46.0% but less than
     46.5%; and (iii) 90% of the Contingent Shares allocable to the first two
     quarters of 1998 so long as the actual Cost of Sales Percentage is greater
     than 46.50% but less than 47.0%.  Thus, in clause (iii) of the preceding
     sentence, if the Selling Parties earned all of the Contingent Shares for
     the first calendar quarter of 1998, the number of Contingent Shares earned
     for the second calendar quarter of 1998 would be that number of Contingent
     Shares required to cause the total number of Contingent Shares earned for
     the first two calendar quarters of 1998 to equal 90% of the total number of
     Contingent Shares allocable to the first two calendar quarters of 1998.

          (b) For each calendar year during the Earnout Period, the Selling
Parties shall be entitled to earn all of the Contingent Shares allocable to such
calendar year if, subject to Section 3(d) hereof, the actual Net Sales of the
Marketing Entities for the calendar year as a whole and the actual Cost of Sales
Percentage for the calendar year as a whole are such that the applicable table
of Schedule A indicates such an Outcome, notwithstanding the fact that the
   ----------                                                             
applicable Outcome for any one or more calendar quarters indicate that less than
100% of the Contingent Shares allocable to such calendar quarter(s) have been
earned.  Any Contingent Shares earned in one calendar quarter during such
calendar year shall in no event be subject to forfeiture or retransfer to
Vistana.

          (c)    (i)  Except to the extent set forth in Section 3(c)(ii) hereof,
     all determinations hereunder with respect to the number of Contingent
     Shares earned shall be made independently for each calendar year during the
     Earnout Period.  Thus, if the Marketing Entities have Net Sales for any
     calendar year during the Earnout Period that are greater than those
     required to earn 100% of the Contingent Shares for such calendar year or a
     Cost of Sales Percentage which is less than that required to earn 100% of
     the Contingent Shares for such year, neither of such results shall have any
     impact whatsoever on Net Sales or Cost of Sales Percentage for any
     subsequent calendar year.  Similarly, neither the failure to achieve any
     level of Net Sales nor any level of Cost of Sales Percentage for any
     calendar year shall have any impact whatsoever on Net Sales or Cost of
     Sales Percentage for any subsequent calendar year.  Any Contingent Shares
     not earned during any calendar year shall be forfeited by the Selling
     Parties (and may not be earned in respect of any subsequent period) and
     Vistana shall have no obligation to deliver any of such shares to any of
     the Selling Parties.


                                      -2-
<PAGE>
 
               (ii) Notwithstanding Section 3(c)(i) hereof, in the event that
     aggregate Net Sales of the Marketing Entities for the 1999 calendar year
     exceed $48,000,000 (such excess Net Sales being referred to as the
                                                                       
     "Excess"), the Selling Parties shall have the right and option (the
      ------                                                            
     "Inclusion Option"), but not the obligation, to include the Excess in Net
     -----------------                                                        
     Sales of the Marketing Entities for the 2000 calendar year upon the terms
     and conditions set forth in this Section 3(c)(ii).  The Selling Parties may
     exercise the Inclusion Option by delivering written notice to Vistana at
     any time prior to or concurrently with the Selling Parties' delivery to
     Vistana of any summary schedule described in Section 7(a) hereof.  If the
     Inclusion Option is exercised, the Outcome for the end of the most recently
     completed calendar quarter during the 2000 calendar year, or the end of the
     2000 calendar year, as applicable, shall be determined by (y) adding the
     Applicable Share (as hereinafter defined) of the Excess to the actual Net
     Sales of the Marketing Entities for the calendar year through the end of
     the most recently completed calendar quarter or the calendar year, as
     applicable; and (z) utilizing the Cost of Sales Percentage resulting from
     the following equation:

                     CSP = (A x 1999 CSP) + (B x 2000 CSP)

     Where:

          CSP      =     the Cost of Sales Percentage to be used for determining
                         the Outcome

          A        =     the quotient of (i) the Applicable Share of the Excess
                         divided by (ii) the sum of the Applicable Share of the
                         Excess and the actual Net Sales of the Marketing
                         Entities for the calendar year through the end of the
                         most recently completed calendar quarter or the
                         calendar year, as applicable

          1999 CSP =     the actual Cost of Sales Percentage achieved for the
                         1999 calendar year

          B        =     the quotient of (i) the actual Net Sales of the
                         Marketing Entities for the calendar year through the
                         end of the most recently completed calendar quarter or
                         the calendar year, as applicable, divided by (ii) the
                         sum of the Applicable Share of the Excess and the
                         actual Net Sales of the Marketing Entities for the
                         calendar year through the end of the most recently
                         completed calendar quarter or the calendar year, as
                         applicable


                                      -3-
<PAGE>
 
          2000 CSP  =    the actual Cost of Sales Percentage achieved for the
                         calendar year through the end of the most recently
                         completed calendar quarter, or the calendar year, as
                         applicable

          For purposes of this Section 3(c)(ii), the term "Applicable Share"
                                                           ---------------- 
     means 25% for determinations relating to the calendar quarter ended March
     31, 2000; 50% for determinations relating to the calendar quarter ended
     June 30, 2000; 75% for determinations relating to the calendar quarter
     ended September 30, 2000; and 100% for determinations relating to the
     calendar year ended December 31, 2000.

     Example:
     ------- 

     Assumptions:

          .  Excess = $8,000,000

          .  1999 CSP = 45%

          .  2000 CSP (through June 30, 2000) = 46%

          .  Applicable Share of Excess = $2,000,000 per quarter

          .  Net Sales of Marketing Entities for the 2000 calendar year through
               June 30, 2000 = $30,000,000

     Accordingly:

          .  A = $4M/[$4M + $30M] or 11.7647%

          .  B = $30M/[$4M + $30M] or 88.2353%

          .  CSP = (11.7647% x 45%) + (88.2353% x 46%) or 45.8823%

          .  For purposes of this example, the Cost of Sales Percentage that
               will apply to the Net Sales of the Marketing Entities of $34M
               ($30M + $4M) is 45.8823%

          (d)    (i)  In the event that (A) for any calendar quarter, Vistana
     and its subsidiaries do not have sufficient Colorado Inventory (as
     hereinafter defined) to enable the Marketing Entities to achieve Net Sales
     in such calendar quarter which equal or exceed the applicable Agreed
     Colorado Sales Volume (as hereinafter defined) and (B) the Selling Parties
     fail to earn 100% of the Contingent Shares allocable to such calendar
     quarter (or calendar year to date), the Selling Parties shall have the
     right and option (the

                                      -4-
<PAGE>
 
     "Substitution Option"), but not the obligation, to substitute Agreed
      -------------------                                                
     Colorado Sales Volume for such calendar quarter for Actual Colorado Sales
     Volume (as hereinafter defined) for such calendar quarter or calendar year
     to date, as the case may be, in each case on the terms and conditions set
     forth in Section 3(d)(ii) hereof.  The Selling Parties may exercise the
     Substitution Option by delivering written notice to Vistana at any time
     prior to or concurrently with the Selling Parties' delivery to Vistana of
     any summary schedule described in Section 7(a) hereof.

               (ii) If the Substitution Option is exercised, the Outcome for the
     end of the most recently completed calendar quarter or calendar year to
     date, as applicable, shall be determined by

                    (w) subtracting from the actual Net Sales of the Marketing
          Entities for the relevant calendar quarter or calendar year to date,
          as applicable, the actual Net Sales of Colorado Inventory of the
          Marketing Entities for such calendar quarter or calendar year to date,
          as applicable;

                    (x) subtracting from the actual Applicable Expenses of the
          Marketing Entities for the relevant calendar quarter or calendar year
          to date, as applicable, the actual Applicable Expenses incurred by the
          Marketing Entities in connection with Net Sales of Colorado Inventory
          for such calendar quarter or calendar year to date, as applicable;

                    (y) adding to the actual Net Sales of the Marketing Entities
          for the relevant calendar quarter or calendar year to date, as
          applicable, the Agreed Colorado Sales Volume of the Marketing Entities
          for such calendar quarter or calendar year to date, as applicable; and

                    (z) adding to the actual Applicable Expenses of the
          Marketing Entities for the relevant calendar quarter or calendar year
          to date, as applicable, the Hypothetical Applicable Expenses (as
          hereinafter defined) for such calendar quarter or calendar year to
          date, as applicable.

               (iii)  For purposes of this Section 3(d), the following terms
     shall have the following meanings:

               "Actual Colorado Sales Volume" means the actual Net Sales of
                ----------------------------                               
     Colorado Inventory achieved by the Marketing Entities for a calendar
     quarter or calendar year to date, as applicable.

               "Agreed Colorado Sales Volume" means the applicable projected
                ----------------------------                                
     "Total Sales" of Colorado Inventory for a calendar quarter or calendar year
     to date, as applicable, set forth on attached Schedule B.
                                                   ---------- 

                                      -5-
<PAGE>
 
               "Colorado Inventory" means vacation ownership interests available
                ------------------                                              
     for sale at one or more resorts which are located within a 15-mile radius
     of Vistana's existing sales center located in Lakeside Terrace, Colorado.

               "Hypothetical Applicable Expenses" means the product of (i)
                --------------------------------                          
     Agreed Colorado Sales Volume for a relevant calendar quarter, multiplied by
     (ii) 40.8%, in respect of the first quarter of a calendar year, 38.2%, in
     respect of the second quarter of a calendar year, 34.3%, in respect of the
     third quarter of a calendar year, or 40.3% in respect of the fourth quarter
     of a calendar year, as applicable.  Hypothetical Applicable Expenses for a
     calendar year to date shall equal the sum of Hypothetical Applicable
     Expenses for all calendar quarters ended prior to the date of
     determination.

          (e) As soon as practical following each calendar quarter during the
Earnout Period (or, if the parties mutually agree, following the close of each
calendar year during the Earnout Period), a determination shall be made (in
accordance with the terms hereof) as to the number of Contingent Shares earned
during such calendar quarter (or during each calendar quarter of such calendar
year, in the case of annual determinations hereunder) and the parties shall
cause the Escrow Agent under the Escrow Agreement to deliver the relevant number
of Contingent Shares to the Selling Parties (acting pursuant to the
representatives appointed under Section 10.15 of the Agreement) or Vistana, as
appropriate.

     Section 4.
     ----------

          (a) For purposes of the foregoing, the term "Net Sales" shall mean the
                                                       ---------                
total net sales volume (less any applicable discounts and/or buyer incentives
deducted from the sales price, which shall also be excluded from the expense
calculation, under generally accepted accounting principles) of the Marketing
Entities (which shall include all sales during the relevant period for which (i)
the relevant paperwork has been signed by the purchaser, (ii) the purchaser has
paid the required down payment so as to qualify for recognition under generally
accepted accounting principles, and (iii) the statutory recision period
applicable to such sales has expired.

          (b) As used herein, "Cost of Sales Percentage" shall mean the quotient
of the aggregate sales, marketing and related operating expenses of the
Marketing Entities outlined in paragraphs 4(b)(i), (ii) and (iii) below
(collectively, the "Applicable Expenses"), divided by Net Sales. The Applicable
Expenses shall consist of the following:

               (i) sales and marketing expenses of the Marketing Entities, which
     shall mean those expenses directly attributable to the sales and marketing
     activities of the Marketing Entities, as generally described in the budget
     attached hereto as Schedule C;
                        ---------- 

               (ii) general and administrative expenses of the Marketing
     Entities, which shall include only those general and administrative
     expenses directly incurred by the Marketing Entities.  For purposes of
     determining the Applicable

                                      -6-
<PAGE>
 
     Expenses, the Marketing Entities shall not be allocated any of the general
     and administrative expenses of Vistana or of any other company or entity
     owned, directly or indirectly, by Vistana; and

               (iii)  the base compensation and any commission, bonus or other
     income of the executive officers of the Marketing Entities, who for these
     purposes shall be Donald J. Dubin, Ronald R. Sharp and David E. Bruce, or
     such other similarly situated individuals hired to perform the sales and
     marketing activities of the Marketing Entities.

          (c) In determining the actual Applicable Expenses of the Marketing
Entities, such determination shall be made before any provision for state, local
or federal incomes taxes and shall generally be made in a manner consistent
with, and shall include or exclude items included in or excluded from, as the
case may be, the target Applicable Expense numbers attached hereto as Schedule
                                                                      --------
C, except the following adjustments shall be made:

               (i) depreciation and amortization shall be computed without
     taking into account any increase in the basis of the depreciable or
     amortizable assets of the Marketing Entities which may result from the
     closing of the transactions contemplated by the Agreement, and any
     increased depreciation or amortization attributable to the transactions
     contemplated by the Agreement shall be disregarded;

               (ii) all expenditures by the Marketing Entities that are required
     or permitted to be capitalized in accordance with generally accepted
     accounting principles shall not be deducted in the year incurred for
     purposes of determining the Applicable Expenses for such year, but instead
     shall be depreciated or amortized over the useful life of the relevant
     asset (in accordance with the depreciation or amortization conventions
     customarily employed by Vistana) and deducted for purposes of determining
     Applicable Expenses based upon such amortization or depreciation schedule;

               (iii)  the moving costs and related expenses (including, without
     limitation, furniture, fixtures and equipment and wall tours) associated
     with the openings of, and transition to, the contemplated new sales centers
     for the timeshare complex at Vail, Colorado and the timeshare complex at
     Scottsdale, Arizona shall be excluded from the calculation of Applicable
     Expenses;

               (iv) all management fees and charges, allocations of Vistana
     overhead or similar expenses or charges whatsoever made by Vistana or any
     of its affiliates, and any other charges made by Vistana or its affiliates,
     against the income or expenses of the Marketing Entities shall be
     disregarded;

               (v) all intercompany receivables or payables between Vistana or
     its affiliates, on the one hand, and one or more of the Marketing Entities,
     on the other

                                      -7-
<PAGE>
 
     hand, and all interest, fees and other charges attributable to such
     receivables or payables shall be disregarded;

               (vi) if after the Closing, the Marketing Entities are required to
     incur travel and development expenses or related costs at the request of
     Vistana and if such costs and expenses are not otherwise related to the
     sales and marketing activities contemplated hereby, then such costs and
     expenses shall be excluded from the determination of Applicable Expenses;
     and

               (vii)  if after the Closing, Vistana makes the decision (for
     whatever reason) to hire and treat as employees those individuals who are
     currently retained by the Marketing Entities on an independent contractor
     basis (except for the executive officers of the Marketing Entities), the
     additional costs associated with such treatment shall be excluded from the
     determination of Applicable Expenses.

          (d) The parties recognize that, upon consummation of  the
Reorganization, the Selling Parties are to have a reasonable opportunity to
cause the Marketing Entities to achieve the target Net Sales levels and Cost of
Sales Percentage and obtain the Contingent Shares, but that Vistana may from
time to time make certain decisions in connection with the operation of the
Marketing Entities, POC and SD that may impact the ability of the Selling
Parties to achieve the target Net Sales levels and Cost of Sales Percentage and
earn the Contingent Shares.  In that regard, Vistana shall endeavor to adopt
policies for the businesses of the Marketing Entities, POC and SD in a manner
consistent with the rights of the Selling Parties to (y) have a reasonable
opportunity to earn the Contingent Shares and (z) manage the day-to-day affairs
of the Marketing Entities, POC and SD in a manner reasonably consistent with
past practices (subject to the terms and provisions of the employment agreements
of the Selling Parties); provided, however, that Vistana shall have the
                         --------  -------                             
discretion to operate the Marketing Entities in any manner that it determines
reasonable if Donald J. Dubin's employment with SCI is terminated for any
reason, other than a termination of Mr. Dubin by SCI without Cause (as such term
is defined in Mr. Dubin's employment agreement); and provided, further that
                                                     --------  -------     
Vistana shall have the discretion to operate POC and SD in any manner that it
determines reasonable if Larry D. Doll's employment with POC is terminated for
any reason, other than a termination of Mr. Doll by POC without Cause (as such
term is defined in Mr. Doll's employment agreement).  Additionally, Mr. Doll
shall work with Vistana to identify and implement integration steps that will
result in more consolidated operations for various support and administrative
functions currently provided in Denver being performed in Orlando or elsewhere.
Vistana acknowledges that certain actions, which are identified below, may
change the nature of the businesses of the Marketing Entities, POC, or SD
causing the Marketing Entities to incur short-term costs that could adversely
impact the ability of the Selling Parties to earn the Contingent Shares.  If
Vistana causes the Marketing Entities, POC or SD to take the following actions
without the consent of the Selling Parties, the Marketing Entities shall
separately account for such actions with reasonable accuracy so as to permit the
determination of actual Net Sales levels and Cost of Sales Percentage as
contemplated hereby, with the effect that the actual Net Sales and Cost

                                      -8-
<PAGE>
 
of Sales Percentage of the Marketing Entities for purposes of the Contingent
Shares calculation shall be restated without consideration of such excluded
costs:

               (i) enter into any business other than the business of timeshare
     or vacation ownership, development, sales and resort management;

               (ii) construct or acquire new resorts or close or shut down
     existing resorts;

               (iii)  borrow funds from any person other than Vistana or its
     affiliates (which loans shall be subject to the provisions of Section
     4(b)(v) hereof);

               (iv) fail to maintain the Marketing Entities, POC or SD as
     separate corporate entities; provided, however, Vistana may maintain the
                                  --------  -------                          
     Marketing Entities, POC or SD as a separate division with separate accounts
     if such separate accounts fairly reflect the separate income and financial
     condition of the division that comprises the businesses of the Marketing
     Entities;

               (v) dispose of any asset of the Marketing Entities, POC or SD,
     the disposal of which materially and negatively affects the ability of the
     Marketing Entities to achieve the Net Sales levels and Cost of Sales
     Percentage hereunder;

               (vi) enter into leases, either as lessee or lessor, except in the
     ordinary course of business;

               (vii)  cause the Marketing Entities to purchase or acquire any
     other business;

               (viii)  sell, merge, consolidate or otherwise dispose of the
     Marketing Entities or all or substantially all of its assets as a going
     concern (other than the sale of inventory sold in the ordinary course of
     business);

               (ix) materially alter the existing sales force or personnel of
     the Marketing Entities;

               (x) cause the working capital of the Marketing Entities to fall
     below those levels reasonably necessary for the operation of the Marketing
     Entities;

               (xi) materially diminish the compensation of any executive of the
     Marketing Entities from that set forth in his employment agreement as of
     the Closing Date;

               (xii)  initiate any policy or practice which the Selling Parties
     can demonstrate to have a material detrimental impact on the their ability
     to earn the

                                      -9-
<PAGE>
 
     Contingent Shares; provided, however that the Selling Parties shall not be
                        --------  -------                                      
     entitled to question any policy or practice if (y) the Selling Parties fail
     to object to such policy or practice within ten (10) days of the receipt of
     a written notice from Vistana that such policy or practice will be adopted
     or (z) the Selling Parties fail to object in a timely manner with respect
     to any policy or practice of which they have knowledge.  The parties
     acknowledge that the modification to the earnout calculations outlined in
     this amended Exhibit 1.5A is intended to provide for the potential impact
     of day-to-day types of items that could have an effect on operations and
     only those items which can be demonstrated to have a material detrimental
     impact will be addressed.

          (e) The Selling Parties acknowledge that, for purposes of calculating
the target Net Sales levels and Cost of Sales Percentage, and achieving the
actual Net Sales amounts and Cost of Sales Percentage, (i) Vistana has not
promised or represented that it will undertake any actions (other than as
expressly provided herein) and the Selling Parties believe (but are not making
any representation or warranty hereby as to actual results) that the target Net
Sales and Cost of Sales Percentage numbers can be achieved based upon the
current inventory of POC and SD (together with the Acquisition Property and any
property or inventory that the Marketing Entities, POC or SD have a right to
purchase, such as the Christie Lodge property) and the existing sales force and
materials of the Marketing Entities (together with the sales centers to be
opened in 1998), and (ii) the Acquisition Property in Scottsdale, Arizona will
be an Embassy Vacation Resort if reasonably possible;

          (f) If Vistana causes the Marketing Entities or POC or SD, if
applicable, to take any of the above listed actions during the Earnout Period
without the written consent of the Selling Parties and the Marketing Entities
cannot separately account with reasonable accuracy for such activities, Vistana
shall cause the Contingent Shares allocable to the calendar year of such actions
to be immediately delivered to the Selling Parties.  In addition, if any such
actions are reasonably anticipated to have a similar effect in subsequent
calendar years, the Contingent Shares allocable to such subsequent calendar
years shall also be immediately delivered to the Selling Parties.

          (g) If during the Earnout Period, Vistana (directly or indirectly by
causing the Marketing Entities, POC or SD to take such actions) (i) terminates
Donald J. Dubin or Larry D. Doll without Cause, as such term is defined in their
respective employment agreements, (ii) deprives Messrs. Dubin or Doll, without
Cause, as such term is defined in their respective employment agreements, and
without their written consent, of the level of authority with respect to the
day-to-day operations of the Marketing Entities, POC or SD commonly attendant to
the executive offices such individuals hold in the Marketing Entities, POC, or
SD (as applicable and established in their respective employment agreements;
provided, however that those costs and expenditures not included in the expenses
--------  -------                                                               
of the Marketing Entities for purposes of calculating the Cost of Sales
Percentage hereunder shall be subject to Vistana's final review and approval) or
(iii) otherwise materially and adversely affect the ability of Messrs. Dubin or
Doll to manage the day-to-day operations of the Marketing Entities, POC, or SD,
Vistana shall cause the Contingent Shares allocable to the calendar year of such
termination or deprivation and to all

                                     -10-
<PAGE>
 
subsequent calendar years during the Earnout Period to be immediately delivered
to the Selling Parties.

     Section 5.  Vistana shall be permitted to increase the number of resorts
     ----------                                                              
being managed and marketed by the Marketing Entities during the Earnout Period;
                                                                               
provided, that upon the addition of each additional resort, all of the revenues
--------                                                                       
attributable to such resort and the following expenses shall be excluded from
the calculation of Net Sales and Cost of Sales Percentage: (a) all of the
expenses directly attributable to such resort (whether pre- or post-opening of
the resort), and (b) an appropriate portion of any indirect expenses of the
Marketing Entities, which shall be determined in good faith by Vistana and the
Selling Parties at the time any new resort is added and which shall be adjusted
from time to time thereafter as appropriate and as agreed to by Vistana and the
Selling Parties.

     Section 6.  In the event of a Change of Control (as herein defined) during
     ----------                                                                
the Earnout Period, the Contingent Shares shall be considered as earned in their
entirety for the remainder of the Earnout Period and for all prior periods of
the Earnout Period.  For purposes of the foregoing, a "Change of Control" means
                                                       -----------------       
(i) any transaction involving the dissolution or liquidation of Vistana other
than a dissolution or liquidation undertaken in connection with a transaction or
series of related transactions described in clause (iii) below, (ii) any sale or
other disposition of one or more of the Marketing Entities to an unaffiliated
third party(ies), or (iii) any (x) sale or other disposition of all or
substantially all of the assets of Vistana to an unaffiliated third party(ies),
(y) merger, reorganization or consolidation to which Vistana is a party and as a
result of which Vistana is not the surviving corporation or becomes at least an
80% owned subsidiary of another unaffiliated corporation, or (z) transaction or
series of transactions pursuant to which one or more unaffiliated third parties
acquire more than 50% of the issued and outstanding Vistana Common Stock, in
each case, if, within twelve months of the effective date of such transaction,
Raymond L. Gellein, Jr. and Jeffrey A. Adler cease to be employed by Vistana (or
its successor) in substantially the same capacity employed as of the date
hereof.

     Section 7.
     ----------

          (a) Promptly upon the determination of the actual Net Sales and the
actual Cost of Sales Percentage for each calendar quarter during the Earnout
Period (or, if the parties mutually agree, following the close of each calendar
year during the Earnout Period), the Selling Parties shall deliver to Vistana a
summary schedule reflecting (i) the determination of the actual Net Sales
pursuant to Section 4(a) hereof; (ii) the determination of the Cost of Sales
Percentage pursuant to Section 4(b) hereof; and (iii) the number of Contingent
Shares earned pursuant to Section 3 hereof based on the Outcome of such actual
Net Sales and such actual Cost of Sales Percentage, and thereafter shall deliver
such supporting documentation as Vistana shall reasonably request.  If Vistana
shall object to the determination of the actual Net Sales, the determination of
the Cost of Sales Percentage, or to the calculation of the number of Contingent
Shares earned by the Selling Parties, Vistana shall notify the Selling Parties
by written notice to the Selling Parties, delivered in accordance with the
notice provisions set forth in Section 10.1

                                     -11-
<PAGE>
 
of the Agreement, within 10 days after notice of such determination is given to
Vistana.  If Vistana and the Selling Parties fail to agree on the actual Net
Sales and/or the actual Cost of Sales Percentage, as applicable, for the period
at issue within 15 days after such objection, the Net Sales and/or the actual
Cost of Sales Percentage, as applicable, for such period shall be examined by
Vistana's independent auditors and by a firm of independent public accountants
of recognized standing selected by the Selling Parties.  Any determination of
actual Net Sales and/or the actual Cost of Sales Percentage for such period
which is agreed to either by Vistana and the Selling Parties or by Vistana's
independent auditors and such other firm of independent public accountants, and
any determination of actual Net Sales by the Selling Parties and/or the actual
Cost of Sales Percentage, which is not objected to as provided in this paragraph
(a), shall be conclusive and binding upon Vistana and the Selling Parties and
their respective successors and permitted assigns.

          (b) In the event Vistana's independent auditors and such other firm of
independent public accountants do not agree on the actual Net Sales and/or the
actual Cost of Sales Percentage for a calendar quarter within 45 days after such
objection to the Selling Parties' determination, such Net Sales and/or the
actual Cost of Sales Percentage shall be examined by a third firm of independent
public accountants of recognized standing selected by agreement of the two
accounting firms, and the report of such third firm of independent public
accountants shall be conclusive and binding upon Vistana and the Selling Parties
and their respective successors and permitted assigns, and shall be enforceable
by a court of competent jurisdiction.

          (c) If Vistana's auditors or any firm of independent public
accountants selected pursuant to paragraphs (a) or (b) of this Section 7 shall
advise Vistana and the Selling Parties that they are unable to determine one or
more issues or amounts necessary to make a determination of the actual Net Sales
and/or the actual Cost of Sales Percentage for the period at issue, each such
issue or amount shall be determined by arbitration by three arbitrators, with
one arbitrator being selected by Vistana and one arbitrator being selected by
the Selling Parties and the third arbitrator being selected by the two
arbitrators.  The decision of a majority of such arbitrators shall be the
decision of the arbitrators and conclusive and binding on Vistana and the
Selling Parties and their respective successors and permitted assigns, and shall
be enforceable by a court of competent jurisdiction.

          (d) Vistana shall bear the fees and expenses of its independent
auditors and any arbitrator selected by it and one-half (1/2) of the fees and
expenses of any third firm of independent public accountants selected pursuant
to paragraph (b) of this Section 7 and any third arbitrator selected pursuant to
paragraph (c) of this Section 7.  The fees and expenses of the firm of
independent public accountants and any arbitrator selected by the Selling
Parties and one-half ( 1/2) of the fees and expenses of any such third firm of
independent public accountants and any such third arbitrator shall be borne by
the Selling Parties.


                                     -12-
<PAGE>
 
     Section 8.
     ----------

          (a) No certificates for fractions of shares of Vistana Common Stock
and no scrip or other certificates evidencing fractional interests in such
shares shall be delivered hereunder.  If the number of Contingent Shares
delivered to a person at any time results in a fractional Contingent Share or
interest therein, the number of Contingent Shares issued to each person shall be
amended up or down (as appropriate) to the nearest whole share.

          (b) In the event of any reclassification, stock split or stock
dividend of or in respect of the Vistana Common Stock after the Closing Date and
prior to delivery of any Contingent Shares in accordance herewith, proportionate
adjustment shall be made in the number or kind of any Contingent Shares which
shall thereafter be delivered hereunder.  In the event the outstanding Vistana
Common Stock is converted, changed, exchanged or reclassified into another
security or form of property pursuant to any merger, consolidation, acquisition
of business and assets, reorganization or recapitalization, if any Contingent
Shares become deliverable thereafter (taking into account the provisions of
Section 6 hereof) there shall be delivered, in lieu of Vistana Common Stock, the
kind and amount of securities or other property into which a share of Vistana
Common Stock was converted, changed, exchanged or reclassified.  This Section 8
shall apply to successive mergers, consolidations, acquisitions, reorganizations
and recapitalization.

          (c) In addition to any rights pursuant to Section 6 hereof, in the
event the outstanding Vistana Common Stock is converted, changed, exchanged or
reclassified (i) into another security or form of property pursuant to any
merger, consolidation, acquisition of business and assets, or other
reorganization in which Vistana is not the surviving corporation, or (ii) into a
non-equity security of which Vistana is the issuer; then Vistana agrees to
deliver, prior to such event, the balance of any Contingent Shares which might
thereafter become deliverable.

     Section 9.
     ----------

          (a) The right to receive the Contingent Shares, if any, to be
delivered pursuant hereto shall not be assignable or transferable except by
operation of law.

          (b) A certificate for any Contingent Shares which become deliverable
shall be mailed, in accordance with the customary practice of Vistana or its
transfer agent, to the Selling Party, or his successor by operation of law, to
whom the Contingent Shares represented thereby are being delivered, at such
person's last known address as provided in the stockholder records of Vistana or
such other address, or in the name of such successor, as shall be furnished in
writing to Vistana by the Selling Party, his duly appointed personal
representative or successor.  Vistana may require proper evidence of succession
and, in any event, shall be fully protected in delivering and mailing
certificates for Contingent Shares to and registered in the name of such person
at such address.

                                     -13-
<PAGE>
 
     Section 10.  During the Earnout Period, Vistana will, and will cause the
     -----------                                                             
Marketing Entities to, make available to the Selling Parties and their
representatives the books and records of the Marketing Entities for purposes of
determining, verifying or contesting any of the amounts relevant to the
determination of the Net Sales (or any component part thereof) and the
determination of the Cost of Sales Percentage (or any component part thereof)
during the Earnout Period or the calculation of the amount of the Contingent
Shares earned by the Selling Parties.

                                     -14-